Neuronal development in the Drosophila eye disc follows a non-clonal mechanism in which the fate of a neuron is determined by cell-cell interactions. The molecular mechanism underlying the development of the R7 neuron has been studied in some detail and involves a signalling molecule, Boss, and a tyrosine kinase receptor, Sevenless. A new member of this pathway, Son of sevenless (Sos), participates downstream of Sevenless, and is a putative activator of Drosophila Ras1. Experiments are proposed to determine if Sos is a substrate for Sevenless and if Sos directly activates Ras1. A sensitive screening technique to detect new genes possibly belonging to the sevenless pathway is also described. Using this method, a number of genes that affect R7 development have been identified. These will be characterized genetically, and the one most likely to have a direct role in the sevenless pathway will be chosen for molecular characterization. Finally, one gene isolated from the above screen is allelic to Star, which has been previously cloned and sequenced. However, the mechanism by which Star affects neuronal development is unknown. Star functions non-autonomously in specifying the fate of the R7 neuron. Experiments are proposed to evaluate the role of Star in this process of neural induction. The long term goal of this study is to genetically dissect a tyrosine kinase pathway that is involved in the development of photoreceptor neurons. The molecules that belong to this pathway bear striking homology to oncogenes isolated in vertebrates. Thus, a detailed analysis of this pathway could, in the long run, provide insights into mechanisms of neural development in the vertebrate visual system.